PLANS OF CIRCULATION. 211 



allows of the introduction of a more powerful me- 

 chanical force of propulsion, derived from the con- 

 tractile power of certain portions of the canal, which 

 are for that purpose enlarged, and endowed with a 

 muscular structure. An organ so constructed con- 

 stitutes, in its more perfect form, a heart. 



The heart is always placed at that part of the 

 stem from which the branches, or arteries, ori- 

 ginate ; and it may be furnished to the stem either 

 of the nutrient (as at h), or of the oxygenising 

 system (as at b) ; or even, in the more elaborate 

 organizations, to both systems at the same time. 



By keeping in view these physiological prin- 

 ciples of construction, we shall be enabled to follow 

 without confusion the steps of gradation which are 

 exhibited in the ascending series of organizations ; 

 and we shall succeed in unravelling the multifarious 

 and intricate combinations which they present in 

 the several classes and orders of the animal 

 kingdom. 



Passing over, for the present, the class of Zoo- 

 phytes, which exhibit only rudimental forms of 

 circulation, we may observe that great diversity 

 exists in the four classes of Articulate Animals 

 with respect to the plan of their circulating system. 

 Among Insects we generally find a single tube, 

 without ramifications, called the dorsal vessel, 

 constituting the only organ subservient to this 

 function : in some instances, however, we may dis- 

 cover that a few vascular branches are continued 

 from this central stem. Such is also the structure 

 in many of the Arachnida; while in one division 

 of that class, possessed of pulmonary cavities, and 

 consequently of a circumscribed and localized respi- 



